Targeting Taste

Researchers are beginning to understand how the tongue's sensors discern sour, bitter, sweet and other flavors -- and it has nothing to do with the famous 'mouth map'

September 29, 2006|By Chris Emery | Chris Emery,SUN REPORTER

Like most natives of Japan, Hiroaki Matsunami has a taste for umeboshi, the super-tart pickled plums Japanese children carry in their school lunchboxes.

Nestled on a bed of white rice, the red orb evokes the sun on the Japanese flag. Nestled on the tongue, it elicits a puckered face.

"I like it, even though I haven't eaten it in a while," said Matsunami, now at Duke University in Durham, N.C., where the potent plums are hard to find. Instead of eating the plums, Matsunami spends his time at Duke trying to figure out how his tongue knows they're sour.

Last month, Matsunami's research team and a separate group of California scientists reported that they've found, at long last, the tongue's tart sensors - molecular docking stations that recognize acidic food particles and send the brain a "sour" message.

The discovery comes eight years into a wave of research into our sense of taste that has answered some long-standing questions and helped dispel at least one myth about the tongue's geography.

Such knowledge may lead to new additives that could make bland foods and unsavory medicines more palatable. It may also explain how tongues, like fingerprints, are unique, making some people sensitive and others numb to certain tastes.

Scientists have long known the surface of the tongue is covered with taste buds - onion-shaped bundles of cells that stand sentry for the digestive system.

Long before the Food and Drug Administration and our modern world of supermarkets and restaurants, taste buds served as a crucial daily survival tool for our ancestors.

The bitter flavor of a wild almond told an ancient human foraging in African forests that the nut was poisonous. A sweet fruit, though, was safe to eat and full of energy. The presence of lions and other man-eaters also added urgency to meal choices.

"If the thing didn't taste good, you might decide it's not worth it to be out in the open and at risk," said Steve Munger, a taste researcher at the University of Maryland School of Medicine.

Much of what we "taste" in everyday life is actually the work of our sense of smell. The tongue can distinguish only five basic flavor sensations - sweet, sour, bitter, salt and umami.

"Bitter is supposed to be a warning that there is something poisonous you should spit out - beer and coffee not withstanding," Munger said.

Umami, the "savory" taste of meat and cheese (also found in the food additive MSG), points to protein. Sweet tells us something is full of carbohydrates. Salt signals, well, that it's salty, and we need a certain amount of salt to survive.

Sour taste comes from acidic compounds, and, like bitter taste, it can be a signal to pass on rotting meat or vegetables. Unpleasant tartness in unripe fruits comes from too much citric acid. It is the plant's way of keeping hungry diners at bay until its seeds mature enough to travel - via the diner's digestive system - to a new home.

Unlike animals, humans have acquired a taste for tartness. In the case of umeboshi, the plums are harvested when still green and sour, before the tree turns them into a tantalizing snack for animals by pumping them full of sugar. (The plums' red hue comes from an herb added during the pickling process.)

A decade ago popular myth held that separate areas of the tongue were responsible for each taste. Referred to as the "mouth map," the concept probably originated with a German text mistranslated into English at the beginning of the 20th century. As recently as 1996, mouth map diagrams still appeared in college neuroscience textbooks.

By the late 1990s, however, scientists had learned enough about molecular biology to turn their sights on taste in a big way. They knew that thousands of taste buds cover the tongue, each containing 50 to 100 taste cells. Each cell has two poles: one end covered with taste receptors projecting from the tongue's surface and one end inside the tongue that connects to the brain via nerves.

When a person bites into an umeboshi, or a lemon, or a kraut-covered hotdog, acidic molecules from the food bump into acid-sensitive taste receptors, activating a taste cell, which sends a sour signal to the brain.

The details of this exchange were long a matter of debate. In line with the "mouth map" concept, some scientists believed the taste buds in different areas of the tongue were filled with cells of only one type: the tip with sweet-sensing cells, the back with bitter-sensing cells, and so on.

Others suggested the location of cells wasn't an issue because each cell could sense all five tastes.